Dispersants are used in engine lubricating oil to prevent sludge formation and to inhibit varnish on hot engine surfaces such as pistons. Hydrocarbon-substituted succinimides are quite effective in such use (U.S. Pat. No. 3,172,892). Likewise, succinimides of hydroxyalkyl substituted amines have been shown to be effective (U.S. Pat. No. 3,219,666). Boronation of such succinimides has also been practiced (U.S. Pat. Nos. 3,322,670; 3,254,025).
Mannich dispersants made from hydrocarbon-substituted phenols, formaldehyde and amines are also known (U.S. Pat. Nos. 3,413,347; 3,725,277; 3,368,972; 3,798,165). Boron-modified Mannich dispersant are described in U.S. Pat. Nos. 3,697,574; 3,703,536; 3,704,308; 3,751,365; and 3,756,953. Fatty acid modified Mannich dispersants are described in U.S. Pat. Nos. 3,798,247 and 3,803,039.